1. Technical Field of the Invention
The present invention relates to optical signal generating apparatus and optical signal generating method that generate an optical signal based on image information. Further, the present invention relates to image forming apparatuses for monochrome and color printers, facsimile equipment, digital copying machines, multi-functional peripherals, etc., using the present optical signal generating apparatus and method.
2. Technical Background
Conventionally, digital copying machines that form images based on the image data of the original obtained by reading out from the original have been realized. In this type of copying machines, the image of the original is read out by a scanner etc., the original image data corresponding to that image of the original is temporarily stored in an image memory. Thereafter, the original image data read out from the image memory is subjected to image processing, and the original image data after image processing is transferred to the printer. In the printer, on a photosensitive drum charged uniformly by a charger, an electrostatic latent image is formed based on the original image data by the exposure section using a polygon mirror, etc.
This electrostatic latent image is developed by a developing unit. The toner image formed on the photosensitive drum by charging, exposure, and development is transferred to the transfer paper (or medium) by the transfer section. The toner image transferred onto the transfer paper is fixed by a fixing unit. As a result, it is possible to form an image based on the original image data on a specific transfer paper, and hence it is possible to copy the original image.
Optical signal generating apparatuses such as LED arrays or liquid crystal device arrays, diffraction grating type light valves, etc. have either been realized or have been proposed as examples of the image writing sections configuring the exposure section. In an optical signal generating apparatus of this type, light modulating elements are arranged in an array direction (typically, in the direction of the main scanning direction) corresponding to a plurality of pixels. Here, a light modulating device is the smallest unit that can be driven individually, and corresponds to each LED element in the case of an LED array or to the area of the LCD defined by one pair of electrodes in the case of a liquid crystal device array. Further, in the case of a conventional diffraction grating type of light valve, the light modulating element is in the form of three or more pairs of a fixed and movable ribbons. Even among these, the diffraction grating type light valves using fixed ribbons and movable ribbons have the advantage that they can be driven at a high speed.
Concerning diffraction grating type light valves, there has been a disclosure that the fluctuations in the optical power in DWDM communication is suppressed using the light diffraction phenomenon in the following Non-Patent Documents.
Non-Patent Document 1: “Suppressing The Fluctuations in The Optical Power in DWDM Communication Using the light diffraction phenomenon,” Oishi et al, Design Wave Magazine 2002 November pp. 86-97
Non-Patent Document 2: “Overview and Applications of Grating Light Valve™ Based Optical Write Engines for High-Speed Digital Imaging,” Trisnadi et al, Paper 5348-05, presented at Photonics West 2004—Micromachining and Microfabrication Symposium
The GLV® (Grating Light Valve™) device described in these documents has been applied in the writing system of a copying machine provided with a high speed image writing function.
FIG. 17 is a perspective view showing a sample configuration of one pixel of the light valve 65′ of the diffraction grating type in a conventional example. The light valve 65′ shown in FIG. 17 can also be seen in the Non-Patent Document 1, and the longitudinal direction of the ribbon is placed along a direction (vertical direction) at right angles to the ribbon array direction. Its surface is formed as a light reflecting layer and every pixel in it comprises a plurality of fixed ribbons 3 and movable ribbons 4 for diffracting the light emitted from the light source. The light valve 65′ of one pixel is configured on the silicon substrate 1 to comprise the common electrode 2, the fixed ribbons 3, and the movable ribbons 4 (electrode). The space 5 is provided between the fixed ribbons 3 and the movable ribbons 4. This space 5 is the area that generates the sagging shape of the movable ribbon 4.
FIG. 18 is a plan view showing an overall configuration of the light valve 65′. The light valve 65′ shown in FIG. 18 has a length Lx in the array direction I. Its width in a direction II perpendicular to the direction I (in the vertical direction) is Wx. In this example, when three fixed ribbons 3 and three movable ribbons 4 are designed to correspond to one pixel, the light valve 65′ is configured so that three pairs of fixed ribbons 3 and movable ribbons 4 are arranged for #N pixels along the line direction. Using a light valve 65′ with such a #1˜#N pixel configuration, the light emitted from the light source is made to be diffracted due to its diffraction phenomenon. Further, in this type of light valves, three or more pairs of fixed ribbons and movable ribbons are assigned for each pixel, and the fixed ribbons and movable ribbons within a pixel are subjected to a common drive.
FIG. 19 is a view of the top surface of the image writing section 60′ configuring the exposure section in a conventional example.
In the image writing section 60′ shown in FIG. 19, the laser light beam L emitted from the laser light source 63 is formed into a parallel light beam using the optical system 64 such as a collimator lens etc. The parallel light beam formed by the optical system 64 enters the light valve 65′. The light valve 65′ operates so that the movable ribbon 4 is modulated based on the image data in units of a line.
At this time, when a writing voltage based on the image data is applied between the movable ribbons 4 and the common electrode 2, the movable ribbons 4 bend towards the common electrode 2 according to that voltage. When the laser light beam enters the diffraction grating with the shape of projections and depressions formed by this bent movable ribbons 4 and the fixed ribbons 3, at the pixels for which a writing voltage has been applied, a first order diffracted light is generated. At the pixels for which no writing voltage has been applied, a 0th order diffracted light is generated or reflected. The first order diffracted light L1 is diffracted with a diffraction angle θ.
In this example, the 0th order diffracted light L0 modulated in units of a line by the light valve 65′ is expanded by the optical system comprising the Fourier transform (FT) lens 66a, the slit 67′, and the inverse Fourier transform mirror 66b etc., and is then impinged on the photosensitive drum 71. A spot shaped opening is formed in the slit 67′. The slit 67′ is placed between the FT lens 66a and the inverse Fourier transform mirror 66b and is positioned at the point where the 0th order diffracted light gets focused. The 0th order diffracted light L0 forms an image on the photosensitive drum 71. Because of this, the image data of one line is simultaneously becomes the 0th order diffracted light L0 and can be exposed (recorded) onto the photosensitive drum 71. As a consequence of this exposure, the latent electrostatic charge image is formed on the photosensitive drum simultaneously for one entire line.
An image recording apparatus using a diffraction grating type GLV® device is disclosed in Japanese Unexamined Laid-Open Patent Publication No. 2000-131628A (Page 6, FIG. 8). According to this image recording apparatus, a light modulating array that includes a diffraction grating type GLV® device is provided. The GLV® device is placed so that its longitudinal direction coincides with the longitudinal direction of the light modulating array. The longitudinal direction of the light modulating array device is made to coincide with the width direction of the rotating drum for recording. When the light modulating array is configured in this manner, it can withstand a high output laser light and it is possible to record the image at a high speed on a heat mode recording medium.
However, when the driving method of the light valve 65′ such as the one shown in Non-Patent Document 1 is used, the image of a slant line is likely to be formed in a jagged manner. FIGS. 20A and 20B show an example of writing a slant line image and an example of driving the light valve 65′.
FIG. 20A is the case when three fixed ribbons 3 and three movable ribbons 4 correspond to one pixel, and is an example in which only 5 pixels in the column direction and five pixels in the row direction in a matrix are selected from the outline part 7 of the slant line image formed based on the image data. The slant line part is the black part in which the image data “1” is written. The white-on-black-background is the white part of the image in which the image data “0” is written. The border between the white part and the black part of the slant line very often becomes “jagged” and has the shape of stairs.
The “jagged” line phenomenon shown in FIG. 20A is the case when the image data corresponding to the outline part 7 of the image is written by the light valve 65′ shown in FIG. 20B, and when a pixel comprising three fixed ribbons and three movable ribbons is made the black part, it is considered as the ON pixel 9 in which the writing voltage is applied to the three movable ribbons 4, and, when a pixel is made the white part, it is taken as the OFF pixel 10 in which no writing voltages are applied to the three movable ribbons 4. In other words, the three pairs of fixed ribbons 3 and movable ribbons 4 constituting a pixel are controlled so that they are driven simultaneously. In such a method, when the light valve is driven, an image with a slant line as shown in FIG. 20A will be formed in a jagged manner. In addition, the jagged line phenomenon described above is a problem that is also present in Patent Document 1. Further, this problem is also present in the usual optical signal generation apparatus having light modulating devices place in a row.
However, apart from the above, if the diffraction grating type light valve 65′ that has been disclosed in Non-Patent Document 1 and in Patent Document 1 is attempted to be used as such in an image forming apparatus of a copying machine, etc., it will be necessary to install an optical system such as a Fourier transform lens 66a, a slit 67′, and an inverse Fourier transform mirror 66b etc., for separating either the 0th order diffracted light L0 or the 1st order diffracted light L1. Such optical systems are both very expensive and complicated, except for the slit 67′, and hinder the achievement of low cost in high speed digital copiers.